Various methods of manufacturing contact lenses are known, including spin casting, machining (for example by diamond turning), and cast molding (for example, using injection molded mold halves). Cast molding of contact lenses involves forming a pair of mold halves (i.e., a first mold half and a second mold half), placing a volume of a contact lens formulation on an optical quality surface of one of the two mold halves, and placing the two mold halves in contact with each other to form a contact lens mold assembly that has a contact lens-shaped cavity containing the contact lens formulation. The contact lens mold assembly is then exposed to conditions to cause the contact lens formulation to polymerize or cure in the contact lens mold assembly.
Contact lenses providing common prescriptions, for example to correct myopia, hyperopia or astigmatism, are mass-produced in very large numbers, at least for low to moderate lens powers. However, it is not economical to produce very large numbers of contact lenses for prescriptions that are less common; contact lenses have only a limited shelf life, so large numbers cannot be produced and stored until they are needed. A preferred approach is to provide such lenses on a “made-to-order” basis.
One approach to producing a made-to-order lens is to cast-mold the lens in a contact lens mold assembly in which at least one of the mold halves includes a surface that has been machined to provide a shape suitable for a desired prescription. For example, WO2010062520A2, describes injection-molded ophthalmic lens mold halves that are subsequently machined to have customized optical quality lens-defining surfaces for producing customized ophthalmic lenses.
US 2007/0036878 A1 is an example of a document describing contact lens mold halves having a “pan-handle” shape; that is, mold halves having a circular head portion and an elongate tail (or “handle”) portion. The circular head portion includes a concave or convex surface which co-operates with a corresponding convex or concave surface of another mold half to form a contact-lens-shaped cavity in which a contact lens is to be cast. The elongate tail portion is useful for handling the mold half and can be used for other purposes, for example to receive a label in order to identify the mold half. The pan-handle contact lens mold halves described in US 2007/0036878 A1 each have both a concave and a convex lens-forming surface, both of which can form contact-lens-shaped cavities in co-operation with adjacent mold halves, but pan-handle contact lens mold halves are envisaged in which only one such lens-forming surface is provided.
During machining, the mold half is rotated at high speed and a cutting tool is brought into contact with the surface of the mold half, with the high-speed rotation resulting in material being cut away by the tool. The optical surface is provided with a customized shape by moving the cutting tool relative to the surface whilst the mold half is rotating. The cut-away material, referred to as “swarf” typically falls away from the cut surface, or is sucked and/or blown away.
However, when attempting to machine a pan-handle contact lens mold half, problems can arise. In particular, the elongate tail portion (i.e. the handle of the pan handle), being part of the mold half, rotates at high speed, in a propeller-like manner. Swarf can readily become entangled with the elongate tail portion, and can then interfere with the cutting process, causing damage to the mold half.